Orthopedic implants for repair of fractured and/or diseased bones presently constitute a major industrial development because of their ability to rehabilitate patient's joints and load-bearing bone members. Many present-day bone implants utilize biologically compatible metal substrates, typically stainless steel, cobalt-chrome alloys or titanium alloys, and others use ceramic substrates which are nonmetallic inorganic materials often in the form of oxides, nitrides, borides, carbide or sulfides. However, such metal and ceramic implants have a modulus far different from that of human bone. It has now been found that there are long-term advantages to the implantation of prostheses having a modulus of elasticity, i.e. Young's modulus, that is closely similar to that of human cortical bone, particularly where an articular surface is involved at a bone joint. The Young's modulus of cortical bone is measured at between about 20 and 27 GigaPascals. Over the past decade or so, there has been increased interest in producing bone implants of pyrocarbon-coated graphite in order to more closely mimic the mechanical properties of human bone. Cortical bone is a dense, solid mass with only microscopic channels; it forms the outer wall of all bones and is largely responsible for the supportive and protective function of the skeleton. Cortical bone has a Young's modulus of about 20 to 30 Giga Pascal (GPa); such can be closely matched by pyrocarbon that has been coated upon graphite substrates. It has been found that in such instances, long-term compatibility is significantly aided by essentially matching the Young's modulus of a bone implant to that of the cortical bone in which it will be implanted and with which it will interface. As such, artificial isotropic graphite coated with dense isotropic unalloyed pyrocarbon has been found to provide an excellent material for the manufacture of such bone prostheses from the standpoint of its biocompatibility and strength and because it can be deposited with a Young's modulus close to that of cortical bone.
A dense surface layer of pyrocarbon can be deposited by a fluidized bed deposition process so as to exhibit wear-resistant, biocompatible, non-thrombogenic properties and a desired Young's modulus. Such pyrocarbon, upon polishing, provides an excellent articular surface for an implant at a bone joint or the like where there will be articulation with native bone and cartilage. Favorable chemical properties of such pyrocarbon, in addition to its matching mechanical properties, creates an excellent articular surface when included as a part of an implant for a bone that is being repaired. However, in those locations on the implant where it juxtaposes with native bone, the inherent characteristics of the pyrocarbon that render it a desirable articular surface may not result in strong joinder to living bone into which it is being implanted.
Orthopedic manufacturers have searched for biocompatible coatings that will improve long-term attachment of metal and ceramic prostheses and have often coated with ceramics, such as hydroxylapatite, and/or with more biocompatible metals, achieving some improvements but less than total satisfaction. However, the unique nature of such pyrocarbon, an essentially organic material, i.e. organic chemistry being the chemistry of carbon, is such that techniques applicable to coating such hard and/or brittle surface do not translate to the coating of a pyrocarbon surface having this desired Young's modulus. Thus, the search has continued for improved coating procedures that can be used to securely anchor and create a coating upon a particular region of a bone implant that has an overall dense pyrocarbon outer surface in order to significantly enhance its secure attachment to living cortical bone at the locations where there will be interfacial contact.